#include<stdio.h>
int main()
{
	int a=0;
	int sum=0;
	double n=-1;
	while(a!=-1)
	{
	sum+=a;
	n++;
	
	scanf("%d",&a);	
	}
	double avarage=sum/n*1.0;
	printf("%f",avarage);
	return 0;
 } 
 
